Enhancement of fermentative hydrogen production from green algal biomass of Thermotoga neapolitana by various pretreatment methods

Biomass of the green algae has been recently an attractive feedstock source for bio-fuel production because the algal carbohydrates can be derived from atmospheric CO2 and their harvesting methods are simple. We utilized the accumulated starch in the green alga Chlamydomonas reinhardtii as the sole substrate for fermentative hydrogen (H2) production by the hyperthermophilic eubacterium Thermotoga neapolitana. Because of possessing amylase activity, the bacterium could directly ferment H2 from algal starch with H2 yield of 1.8–2.2 mol H2/mol glucose and the total accumulated H2 level from 43 to 49% (v/v) of the gas headspace in the closed culture bottle depending on various algal cell-wall disruption methods concluding sonication or methanol exposure. Attempting to enhance the H2 production, two pretreatment methods using the heat-HCl treatment and enzymatic hydrolysis were applied on algal biomass before using it as substrate for H2 fermentation. Cultivation with starch pretreated by 1.5% HCl at 121 °C for 20 min showed the total accumulative H2 yield of 58% (v/v). In other approach, enzymatic digestion of starch by thermostable α-amylase (Termamyl) applied in the SHF process significantly enhanced the H2 productivity of the bacterium to 64% (v/v) of total accumulated H2 level and a H2 yield of 2.5 mol H2/mol glucose. Our results demonstrated that direct H2 fermentation from algal biomass is more desirably potential because one bacterial cultivation step was required that meets the cost-savings, environmental friendly and simplicity of H2 production.